The polar vortex of 2014 will test the cold hardiness of trees and vines.

In this satellite image, a frontal system that brought rain to the East Coast in early January is draped from north to south. Behind it lie the clearer skies that brought bitter cold air associated with the polar vortex. Image captured by NOAA’s GOES East satellite on January 6, 2014. CLICK TO ENLARGE

Mother Nature opened the new year with an arctic blast that drove temperatures across the Midwest and East to levels not seen since 1994.

It brought some new terminology to the TV weather shows—who’d ever heard of a polar vortex?—and it revived cold hardiness as a hot topic of conversation. University fruit experts across the northeast quadrant of the United States were e-mailing each other, comparing notes.

Twenty years may not seem like a long time, but much has changed in orchards and vineyards over the last two decades—new varieties, new rootstocks, and new growing systems. And, especially in vineyards, the general trend to ever-warmer winters encouraged wine grape growers to venture further into the cold-tender viniferous grapes to make better wine than they can from the French hybrids.

This winter may test the results of their boldness.

In only a few fruit-growing areas did temperatures fall to levels known to be damaging. One of those was southwest Michigan, where temperatures reached -20˚F on some sites.

Mark Longstroth, the extension fruit educator in that area, said that in his experience minus 13 degrees F will damage peaches and by minus 16, damage will be so extensive growers won’t bother to spray what’s left. Coincidentally, he began work in southwest Michigan in 1994, so he remembers the last really cold winter.

“I expect there will be a light peach crop in our area this year,” he said, “especially on sites away from Lake Michigan.”

“Peaches are the simplest to understand,” he said. “They have a single flower, and they’re quite cold sensitive. Apples are more complex—they have a bunch with five flowers—and most apple varieties are safe to minus 40.”

With peaches, when temperatures reach minus 20 degrees F, the tree wood itself can be damaged, and damaged trees will show signs of water stress the next summer and begin to decline and die.
It’s not just temperature that makes the difference, however.

Cold ​temperatures have different effects at different times, and how rapidly the temperature changes makes a difference, too.

Bill Shane, the peach breeder at Michigan State University’s Southwest Michigan Research and Extension Center, noted in an e-mail that Michigan was virtually wiped out of the peach world in 1906 when temperatures fell to 10˚F. That’s not even below zero, but it occurred on October 10, before trees had hardened off, and it killed 73 percent of the state’s peach trees.

On January 16 and 19, 1994, temperatures fell to minus 17 to minus 22 degrees F, resulting in nearly complete crop loss and widespread damage to wood. This year looks similar.

Pennsylvania

In Pennsylvania, Penn State University tree fruit professor Dr. Bob Crassweller said the amount of the temperature drop and the speed of it is a greater concern than the absolute low temperature.

Temperatures fell to nearly minus 9 degrees F in some areas, but not in Adams County, where most of the fruit is grown. But when temperatures falls 40 degrees or more in a day, that’s a cause for concern.

As Longstroth explained, cold tolerance is complicated. Basically, he said, warm temperatures in winter may not break dormancy, but they can “reset” the cold hardiness level.

“If the temperature has been above freezing recently, the plants have lost cold hardiness,” he wrote in an e-mail.

His rule-of-thumb is that after two days with night-time lows above freezing, the plants will have lost all their acclimation to cold weather and be back to the 0-degree F damage threshold.

“The zero threshold is only for cold-tender plants such as peaches, wine grapes, and blueberries. More cold hardy are cherries and European plums, with apples and pears being the most hardy. Temperatures of minus 20 to minus 25 degrees F will have an impact on Prunus, and apples and pears should be able to go to minus 25 with little damage.

Longstroth said, “If the high or low temperatures drop more than 50 degrees, I worry; 70 degrees, I really worry. This means we had a lot of free water in the plant (or as the growers would say, the sap is up), and it froze real fast and I think damage is likely.”

“This is the second coldest blast in my 20 years here in Michigan. In 1994, we got to minus 24 to minus 28 degrees F in February. It was so cold the brine was freezing in the pickle vats. We saw many young trees killed outright. Wine grapes killed to the snowline. Tart cherries that were defoliated early died and did not leaf out.”

He said there were no peaches, and the lower trunks above the snow, where the air is coldest and the temperature differential is the greatest, were damaged. Older peaches and plums began to lose leaves during the first dry spell the following season and declined for several years after. Many juice grapes leafed out normally and then collapsed during the summer.

“We had dead flower buds in all crops including apples, but other than peaches, wine grapes, and ­blueberries, the state crops were normal,” he recalled.

Crassweller noted that wine grape growers will have their vineyards tested by the cold weather. “We’ve been pushing the envelope on some varieties,” he said. Some growers who have planted tender vinifera varieties realize they are at the margin, so they cover their graft unions in the fall to protect at least the base of the scion variety.

Still, if grapes are killed back to the rootstock, it will take two to three years to reestablish the vine, and the vineyard will never recover its ­uniformity, he said.

In western New York State, extension educator Deb Breth said temperatures had not gotten as cold along Lake Ontario as further south. The polar vortex swept cold air south on the west side of the Great Lakes, and then turned it east along the Ohio River Valley.

In southwest Michigan, which is normally protected by Lake Michigan, the cold air was actually coming in from the south and east and not crossing the big lake. It was colder in southwest Michigan than it was in northern fruit areas along the lake.

Chris Doll, a long-time industry observer in Illinois who retired from the University of Illinois extension program, said in an e-mail, “It is known that resistance and acclimation are two factors that affect freezing injury. To me, the classic resistance is that of lambrusca grapes, when compared with vinifera and some of its hybrids. For that reason, I have concerns about the Midwest grape and wine industry that has not seen the damaging minimum temperatures in recent years.

“And, for all of the new Prunus interspecies and varieties that have been touted as cold hardy, this might be the test, and I hope that my pessimism is for naught.”

The frigid weather may have chilled some people’s thoughts about global warming. Fear not. The polar vortex, which slid south across North America from the Arctic, also slid north in Europe and Asia on the other side of the world. The high temperature in Sparta, Michigan, at 4 p.m. on January 7 was 7˚F, while in Moscow, Russia, it was 34˚F at the same time (1 a.m. there). That means the Michigan high of the day was 27 degrees colder than the Russian low.

“I hope that this is just another of the worrisome cold spells that growers must go through, and that the remainder of the 2013-2014 winter is less stressful,” Doll said, expressing the consensus.

But after the very cold spell the first week of the year, Michigan temperatures rose to above freezing on January 10 and were expected to fall again the following week. “A roller coaster, that’s the concern,” Crassweller said. •

After growing up on a Michigan dairy farm, Richard Lehnert began writing about farming in 1962, while still a junior studying journalism at Michigan State University. He worked at newspapers for a year before joining the staff of Michigan Farmer, where he spent 26 years, the last 15 as chief editor. He joined the staff of Good Fruit Grower in 2010. Based in Michigan, he primarily provides articles about fruit production from the eastern side of North America.